Method for forming cornered images on a substrate and photomask formed thereby

ABSTRACT

A method for forming square shape images in a lithographic process is disclosed wherein a first plurality of lines running in a first direction is defined in a first, usually sacrificial, layer, and then a second resist is defined wherein the lines run in an intersecting pattern to those of the first layer, thereby creating cornered images wherever the first and second layer intersect and in the open areas between the lines. Methods are proposed for developing the square intersecting areas and the square angle areas defined by the openings. Additionally, a photomask is disclosed in which the length and width of the cornered images are independently patterned using the two-exposure process.

This application is a divisional of U.S. application Ser. No. 08/917,009filed Aug. 21, 1997, now U.S. Pat. No. 5,959,325 issued Sep. 28, 1999.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to the manufacture of integratedcircuit (IC) chips and, more specifically, to a method for formingimages having sharp corners during lithographic processing by exposingthe feature in two angularly intersecting directions and a photomaskformed thereby.

2. Background Art

Corner rounding and image foreshortening are problems for both photomaskfabrication (especially for laser writers) and also for waferfabrication. This problem, as depicted schematically in FIG. 1, isusually observed for small image shapes which are near the resolutionlimit of the lithographic exposure tool. In mask fabrication, forexample, large features print with a shape that closely approximates thedesigned image. The length and width of the large shapes matches thedesigned values, with a slight rounding of the corners of the pattern.This corner rounding can be caused by the reticle etch process, whichfor chrome films has historically been an isotropic, wet process. It canalso be caused by the limitations in the resolution of the writing beamof the reticle expose tool, whether e-beam or optical laser. For theprinting of small images, which are at or near the resolution limit ofthe expose took, the corner rounding becomes more pronounced, and thelength of the image can be severely shortened relative to its designedlength. This can be caused by several factors, including the loss ofedge acuity of the aerial image at the resolution limit of the exposetool, the variation in effective exposure dose for small images relativeto larger images, and the degradation of the aerial image by chemicaldiffusion processes in the photoresist after expose. These problems canbe compounded for lithography on the wafer because the reticle roundingand shortening effects are combined with the tool and process componentsfrom the wafer processing.

Image shortening and rounding can significantly affect the ability toscale devices to smaller groundrules. For example, the capacitance of aDRAM device is directly related to the area of the storage node. As DRAMdevices are scaled to smaller dimensions, the corner rounding andshortening effects reduce the capacitance values which can be attained,and constrain the ability to scale the density of the DRAM array. Inanother example from DRAM, the overlap of a strap connection betweenstorage node and diffusion areas can be limited by the rounding of strapand storage node during the lithographic process. The rounding of thestrap and storage node pull the features away from each other so thatthey fail to intersect and make an electrical connection. In order toprint the features large enough that the rounded corners intersect, anelectrical short defect is created at other sections of the images whererounding and shortening effects are not observed, and the over-sizedpatterns fuse together.

Modeling has shown that a large percentage of the image shortening isdue to mask corner rounding in the 64 Mb dynamic random access memory(DRAM) design, and becomes more pronounced for the 256 Mb DRAM design.

In the present era of very large scale integration and ultra large scaleintegration, new techniques are continuously being developed to moreefficiently utilize the space within semi-conductor devices whilemaintaining or improving present production efficiency.

As IC dimensions continue to shrink, printed lithographic features withminimal curvature are critical to achieve the packing density requiredto obtain the desired cell size.

The larger the curvatures on the images, the larger an area has to beallotted for two intersecting images. Therefore, printing of smallrectangles with minimal foreshortening is becoming a difficult problemfor the process fabricators as the dimensions of the structures becomesmaller.

Traditionally, masks have been fabricated with a single layer process inwhich a beam spot is rastered across an image to form a pattern. Thistechnique inherently leads to corner rounding problems depending on thebeam spot size. Smaller spots minimize the corner rounding problems, butthese problems are solved at the cost of writing time and edgesmoothness. Foreshortening is also becoming an increasingly largeproblem at the mask level, as can be seen in FIG. 2.

What is known as the “k” factor is defined in the Rayleigh model forlithographic resolution, in the equation:

R=kλ/NA

where R is the resolution, k is an empirically derived parameter that isdependent on photoresist performance, λ is the exposure wavelength, andNA is the numerical aperture of the expose tool. Presently, improvingthe “k” factor and reducing the wavelengths of the exposure have beenthe subject of much research, in order to improve resolution as featuresize continues to decrease.

Issues relating to corner rounding and image foreshortening are becomingmore acute as one uses lower “k” factor, i.e., lower fidelitylithography processes, in both the mask process and the wafer process inorder to make increasingly smaller features. As is shown FIG. 2 in thenew foreshortening data, as the design width decreases below 0.35microns (μm) the foreshortening becomes more pronounced.

SUMMARY OF THE INVENTION

The present invention is a method of forming cornered images on asubstrate comprising the steps of: (a) providing a substrate having afirst layer of selectively etchable material thereon; (b) forming aplurality of parallel edged openings in the first layer of etchablematerial, the openings aligned to form par is of straight-edged firstregions; (c) depositing a layer of selectively etchable material overthe openings in said first layer of etchable material; (d) forming asecond plurality of parallel edged openings in the layer of patternablematerial; said second plurality of openings intersecting adjacent pairsof said straight-edged first regions forming a plurality of secondregions bounded by two edges of one of said first regions and two edgesof one of said second region; and (e) processing the substrate in thesecond regions defined by said first and second openings.

In order to compensate for foreshortening problems, including curvaturein the masks and printing on the wafers, the present invention providesa two-exposure method to decrease the amount of foreshortening in thephotoresist. The present invention solves the problems related to imageforeshortening by printing a first line in a first direction and asecond line in a direction angularly offset from the first direction,thereby forming cornered images, typically rectangular or square-shapedimages, at areas of intersection. The problems related to imageforeshortening and corner rounding are either completely eliminated orreduced to a point where the printed image is acceptable within designparameters.

For example, the photomask of the present invention may be made using astandard, blank mask comprising a radiation blocking material, such aschrome, on a transparent substrate, such as fused silica or quartz,would be provided. Next, a hardmask material or other sacrificial layerthat is relatively thin would be deposited over the entire surface. Afirst layer of a photoresist material is deposited, exposed to a firstpattern, and developed on the surface of the hardmask in the mannercommonly practiced in the art. The hardmask material is then etched,down to the chrome layer. The first layer of photoresist material isthen stripped. A second layer of a photoresist material is deposited, asecond pattern, having lines running in a second pattern, is alignedwith the first pattern so that the lines of the second pattern angularlyintersect with the lines of the first pattern. The second layer ofphotoresist material is exposed to the second pattern, and developed.The chrome is then etched down to the substrate. In this way, theforeshortening and curvature problems are minimized. Various alignmentmethods as well as phase-shifting structures may be utilized inconjunction with this method.

In another example, the present invention may include some features,such as deep trench (DT) capacitor structures and isolation trench (IT)structures, it is desirable to provide square shapes withoutforeshortening, especially as scaling and design constraints force thesefeatures to become smaller and smaller. In order to accomplish thesegoals, this example of the present invention provides three methods offorming square shapes for these features.

In the first of these exemplary methods, a grating pattern is used witha two-exposure process with a negative tone resist to form islands ofsquare open areas separated by solid lines of resist. This processcomprises the steps of: providing a substrate; depositing a first layerof resist; exposing the first layer of resist to a first pattern havinglines in a first direction; developing the first layer of resist;depositing a second layer of resist; exposing the second layer of resistto a second pattern having lines in a second direction, orthogonal tothe lines in the first pattern; developing the second resist; andetching the substrate. Optionally, a “memory layer” or thin sacrificiallayer may be used to memorize the first lithographic pattern, allowingthe stripping of the first resist. This alleviates topology and depth offocus issues with the second printing because the memory layer may bemuch thinner than a typical layer of resist as long as the etch rateratios are favorable. Care must be taken with the alignment of the firstand second layers. The etched open areas are the logical “OR” of thelithographic open areas.

In a second exemplary embodiment of the process for use with IT and DTfeatures some areas are printed with the square or otherangularly-cornered images, while other areas are protected, i.e., thematerial is selectively removed from the areas that will be angularlycornered by printing a first set of lines in a first direction, therebyremoving a first layer of material in the exposed portions. A second setof lines in an angularly intersecting pattern is then printed. In theareas of intersection the material may then be etched to form squareholes. The etched open areas are the logical “AND” of the lithographicopen areas.

Typically, a silicon wafer substrate is provided. A first layers ofpolysilicon (polySi), silicon nitride and oxide typicallytetraethylorthosilicate (TEOS), are deposited. These are the layers thatneed to be patterned.

Second, thin layers of polysilicon, silicon nitride and TEOS,respectively, are then layered onto the substrate surface to act assacrificial masking layers. The order of the thin layers is chosen to bethe same as the thick underlying layers, therefore allowing in situstripping of the thin, masking layers. A first layer of resist isdeposited, patterned, and the second layer of oxide is etched selectiveto the nitride. The first layer of resist is then stripped and a secondlayer of resist is deposited. The second layer of resist is thenpatterned with the lines running orthogonal to those of the first layerof resist. The exposed portions of the second layer of nitride is thenetched, selective to both the second layer of polySi and the secondlayer of TEOS, thereby exposing portions of the second layer of polySi.This then forms the pattern to be selectively etched all the way intothe substrate by etching each subsequent sharply-cornered area selectiveto the other materials.

In a third exemplary embodiment of the process for use with IT and DTfeatures, the first and second layers do not form a complete gratingpattern. The additional layers are incorporated to form a non-grating or“dog bone” pattern, which is used for the second pattern to guaranteecornered shapes.

In these manners, cornered images with fewer problems with respect tocorner rounding and image foreshortening are produced.

BRIEF DESCRIPTION OF THE DRAWINGS

For an understanding of the nature and objects of the present invention,reference should be made to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic of image foreshortening in lithographic printingof square shaped images;

FIG. 2 is a graphical representation of the problem of imageforeshortening as a function of dimension;

FIG. 3 is a top-down schematic of a first step in a process forproducing a photomask in accordance with a first embodiment of thepresent invention;

FIG. 4 is a cross-sectional schematic across lines 4—4 of FIG. 3;

FIG. 5 is a top-down schematic of a second step in the process forproducing a photomask in accordance with the first embodiment of thepresent invention;

FIG. 6 is a cross-sectional schematic across line 6—6 of FIG. 5;

FIG. 7 is a top-down schematic of a third step in the process forproducing a photomask in accordance with the first embodiment of thepresent invention;

FIG. 8 is a cross-sectional schematic across line 8—8 of FIG. 7;

FIG. 9 is a top-down schematic of a fourth step in the process forproducing a photomask in accordance with the first embodiment of thepresent invention;

FIG. 10 is a cross-sectional schematic across line 10—10 of FIG. 9;

FIG. 11 is a cross-sectional schematic across line 11—11 of FIG. 9;

FIG. 12 is an alternative cross-sectional schematic across line 11—11 ofFIG. 9;

FIG. 13 is a cross-sectional schematic of a first step in a secondembodiment of the present invention;

FIG. 14 is a cross-sectional schematic of a second step in a secondembodiment of the present invention;

FIG. 15 is a top-down schematic of a third step in a second embodimentof the present invention;

FIG. 16 is a top-down schematic of a fourth step in a second embodimentof the present invention;

FIG. 17 is a cross-sectional schematic taken across line 17—17 of FIG.16;

FIG. 18 is a cross-sectional schematic taken across line 18—18 of FIG.16;

FIG. 19 is a top-down view of a fifth step in a second embodiment of thepresent invention;

FIG. 20 is a cross-sectional schematic taken across line 20—20 of FIG.19;

FIG. 21 is a cross-sectional schematic taken across line 21—21 of FIG.19;

FIG. 22 is a top-down view of a sixth step in a second embodiment of thepresent invention;

FIG. 23 is a cross-sectional schematic taken across line 23—23 of FIG.22;

FIG. 24 is a cross-sectional schematic taken across line 24—24 of FIG.22;

FIG. 25 is a top-down view of a seventh step in a second embodiment ofthe present invention;

FIG. 26 is a cross-sectional schematic taken across line 26—26 of FIG.25;

FIG. 27 is a cross-sectional schematic taken across line 27—27 of FIG.25;

FIG. 28 is a top-down view of an eighth step in a second embodiment ofthe present invention;

FIG. 29 is a cross-sectional schematic taken across line 29—29 of FIG.28;

FIG. 30 is a cross-sectional schematic taken across line 30—30 of FIG.28;

FIG. 31 is a cross-sectional schematic of a first step in a thirdembodiment of the present invention;

FIG. 32 is a cross-sectional schematic of a second step in a thirdembodiment of the present invention;

FIG. 33 is a top-down view of a third step in a third embodiment of thepresent invention;

FIG. 34 is a cross-sectional schematic taken across line 34—34 in FIG.33;

FIG. 35 is a top-down view of a fourth step in a third embodiment of thepresent invention;

FIG. 36 is a cross-sectional schematic taken across line 36—36 in FIG.35;

FIG. 37 is a top-down schematic of a fifth step in a third embodiment ofthe present invention;

FIG. 38 is a cross-sectional schematic taken across line 38—38 in FIG.37;

FIG. 39 is a cross-sectional schematic of a sixth step in a thirdembodiment of the present invention;

FIG. 40 is a cross-sectional schematic of a first step in forming atrench capacitor configuration, in accordance with a preferredembodiment of the present invention;

FIG. 41 is a cross-sectional schematic of a second step in forming atrench capacitor configuration, in accordance with a preferredembodiment of the present invention;

FIG. 42 is a cross-sectional schematic of a third step in forming atrench capacitor configuration, in accordance with a preferredembodiment of the present invention;

FIG. 43 is a cross-sectional schematic of a fourth step in forming atrench capacitor configuration, in accordance with a preferredembodiment of the present invention; and

FIG. 44 is a cross-sectional schematic of a fifth step in forming atrench capacitor configuration, in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment of the present invention, a first layer ofphotoresist is used to photolithographically define a pattern of linesin a first direction in an optional sacrificial layer. The term “define”as used herein includes: exposing and developing the photoresist;etching the layer beneath the photoresist; and stripping the photoresistfrom the surface. Next, a second photoresist layer is used tophotolithographically define a plurality of lines running in a directionangularly offset from those in the first direction, while leaving theresist protected portions of the sacrificial layer material in place.The substrate is then etched and open square areas of varying sizes,i.e., with independently determined lengths and widths, are formed. Inthis manner, the logical “OR” of the two-step lithographic procedure isetched.

Alternatively, the pattern of lines could merely be defined in the firstphotoresist layer without utilizing a layer of a sacrificial material,however this would be subject to more issues with respect to depth offocus and topology. The second photoresist layer could be defined asdescribed above. Then the pattern formed by the first photoresistmaterial and the second layer of photoresist material could be used as amask to etch into the substrate.

The following example is presented for the purposes of illustrationonly, and are not intended to be limiting as many other possibilitieswould be obvious to one skilled in the art.

The invention results in lithographically square images—that is, imagesin a transfer film (such as photo resist or polyimide) having sidewallsthat intersect at sharp-edged corners (i.e. corners without significantcorner rounding and/or images without significant image foreshortening)in which at least one portion of the image is at or near the resolutionlimit of the photoexposure tool. Using conventional directional etchingtechniques, these images in a transfer film (such as photoresist orpolyimide) are then transferred to underlaying semiconductor layers. Asa result, sidewalls that intersect at approximately ninety degrees canbe defined in these layers, to create a host of structures (e.g. contactvias interconnecting a metal line to another metal line or to a dopedsilicon structure such as doped polysilicon or a source/drain diffusion;a .pillar. of doped semiconductor to define an electrode of a DRAM cell;or an aperture through a photomask) having sharp-edged corners

EXAMPLE 1

To produce an acceptable photomask, having features with sharp, 90°angles at the corners a first adaptation or embodiment of the presentinvention is utilized. A grating pattern is used with a two-exposureprocess with a negative tone resist to form islands of square open areasseparated by solid lines of resist.

As shown in FIG. 4, a substrate 112 having a top surface 114 and abottom surface 115 is provided. The substrate 112 is made from amaterial that will transmit electromagnetic radiation or other suitableactinic energy of the wavelength used in lithography (hereinafter“actinic radiation”), i.e., e-beams, ion beams, or optical radiation,typically having wavelengths in the range from about 190 to about 450nanometers (nm). Fused silica is the preferred material for substrate112, although other materials having a suitable refractive index andother characteristics may also be used. The thickness of substrate 112will vary based on known design considerations. However, in oneembodiment of the present method, the substrate has a thickness of about2-7 millimeters (mm).

Top surface 114 is covered with a blocking layer made from a material120 having an upper surface 124 that substantially blocks thetransmission therethrough of electromagnetic radiation of thepredetermined range of wavelengths. Suitable materials for blockinglayer 120 include those that have mechanical durability and opacity inthin layers, for example: chromium, molybdenum, aluminum, tungsten,titanium, molybdenum silicide, or carbon. Chromium is the preferredmaterial for layer 120 due to its relatively high mechanical andchemical durability and because there is a large installed base ofsophisticated chromium mask repair tools. The specific thickness ofblocking layer 120 will vary as a function of the ability of thematerial used as the layer to block the transmission of radiation.However, when layer 120 is made from chromium, it is preferred that thelayer have a thickness ranging from about 1 nm to about 1,000 nm with athickness of about 100 nm being preferred.

A layer of hardmask material 130 having a first surface 134 and a secondsurface 135 is deposited on the upper surface 124 of the blocking layer120. This hardmask material is used as the etch mask for underlyinglayers. The thickness of this layer is chosen by etch rate ratio to theunderlying layers. The hardmask material is typically a layer of eithersilicon nitride or silicon dioxide. For the purposes of discussionherein, the hardmask material will be referred to as the “nitride layer130” although it is recognized that other materials could have beenchosen.

A layer of a first photoresist material 140 is then deposited on thefirst surface 134 of the nitride layer 130, and the resultant structureis shown in FIG. 3. The first photoresist material is a material thatselectively reacts when exposed to light, thereby changing thesolubility characteristics of the material to either increase thesolubility in areas that have been exposed to the actinic energy source(positive tone) or decrease the solubility (negative tone). In thisparticular example, the choice of positive or negative tone resist isimmaterial, the resist characteristics with regard to processingconditions and sharpness of features being determinative. Thephotoresist material 140 is then exposed and developed in a mannercommonly known in the art. The entire structure is etched and the resistis stripped to form the structure shown in FIG. 5. The process ofexposing and developing the photoresist material, etching the structureand then stripping the resist material will hereinafter be referred toas “defining” the desired features.

As shown in FIG. 5, the features are usually lines running in a firstdirection. In this case they are oriented in a north-south direction. Asshown in FIG. 6, there are areas from which the nitride layer has beencompletely removed, exposing the upper surface 124 of the blockingmaterial 120 and areas in which the nitride layer 130 remains, thesebeing the areas that were protected by the photoresist during theetching step.

A second layer of photoresist 150 is then deposited, exposed anddeveloped to form the structure depicted in FIG. 7. Lines of photoresistmaterial 150 are formed running orthogonally to the lines of nitride130. Portions of the layer of blocking material 120 are exposed on theupper surface 124. The exposed portions have square corners, without thecorner rounding or fore-shortening shortening problems typicallyassociated with square features. As shown in FIG. 8, the lines ofphotoresist 150 are on the upper surface 124 of the layer of blockingmaterial 120.

The exposed portions of the layer of blocking material 120 are etcheddown to the top surface 114 of the substrate 112. The photoresist isthen stripped, and the final mask 160, as shown in FIG. 9, is produced.As shown in FIG. 9, the lines of blocking material 120 run east-west andlines having a hardmask material on the surface run north-south. Asshown in FIGS. 10 and 11, the lines of blocking material simply lie onthe top surface 114 of the substrate 112. As shown in FIG. 12, rim-typephase-shifters may be formed by non-directionally etching edges 122 ofthe lines of blocking material 120. Because the edges are pulled backfrom the edge of the hardmask material 130, the layer of hardmaskmaterial may function as a rim-type phase shifter as long as the overetch process is carefully controlled to create the proper pull-backdistance, depicted as distance Y in FIG. 12.

In a second embodiment of the present invention, the areas that are tobe made angularly cornered are the areas where both the first etchingstep and the second etching step overlap. The areas of overlap may beconsidered the logical “AND” areas. This embodiment utilizes a firstsacrificial layer and a second sacrificial layer to obtain the sharplycornered image in the underlying layer or layers. A first pattern oflines running in a first direction is etched into the first sacrificiallayer of material. Then, a second pattern of lines is defined and etchedinto the second layer of sacrificial material. The etching of the secondlayer must be selective to both the resist material, the material underthe second sacrificial material (if any) and the first sacrificial layerof material. For example, if a polysilicon layer is going to bepatterned, a first layer of silicon nitride may be the first layer ofsacrificial material and a second layer of silicon oxide may be thesecond layer of sacrificial material. The nitride layer could then bepatterned into lines and spaces. Through the spaces, the oxide layerwould be exposed. Next, another photolithographic step would be used topattern the resist and wherever the spaces in the resist pass over thenitride lines, the nitride would remain, however, wherever the spaces inthe second resist pass over areas of spaces in the nitride layer, theoxide beneath would be exposed, and therefore, could be selectivelyetched to the layer of polyimide beneath.

Certain constraints exist in choosing the thicknesses of the variouslayers. For example, the depth of focus and other lithographicconstraints require that the layers be fairly planar when printed. Forthat reason, the preferred embodiments of the present invention chooseto utilize additional, thin layers (and therefore additional processingcomplexity and costs) in order to print the second set of lines withaccuracy even though the surface is either not completely planar or theresist thickness varies.

Other constraints exist in determining which layers may be placed whereas well as how thick they must be. For example, certain materials willetch in the same etchants, however, they etch at different rates. So, inorder to selectively etch through, say the nitride layer, the layerbeneath it must not be etched at the same rate as the nitride in thechosen etchant, or the layer beneath (or above) must be made thicker tocompensate for the etching rates. The following examples are in no waymeant to be limiting, but are exemplary of the basic concepts introducedby this invention.

EXAMPLE 2

In a second embodiment of the process for use with features to beprinted on wafers, for example DT features, some areas are printed withthe square shaped images, while other areas are protected.

In the first of these methods, a grating pattern is used with atwo-exposure process to form islands of square open areas separated bysolid lines of resist. More specifically, a substrate is provided,typically a P-type silicon wafer.

The wafer is preferably coated with a negative tone resist, for example,poly(3-hydroxystyrene), available from Hoechst Celanese of CorpusChristi, Tex., poly(4-hydroxystyrene), poly(4-hydroxystyrene) with oneor more alkyl substituents on the aromatic ring, poly(3-hydroxystyrene)with one or more alkyl substituents on the aromatic ring, or any ofthese as the major subunit in a copolymer, such as PHM-C, commerciallyavailable from Maruzen America of New York, N.Y.

This negative tone resist is then exposed under appropriate exposureconditions to cause lines in either the x-direction or the y-directionto be formed upon developing. One of two processes may then beimplemented. The negative tone resist may be exposed in the oppositedirection, and subsequently both the first set of lines and the secondset of lines may be developed or the first set of lines may be developedand the entire surface of the wafer is again coated with a second layerof negative tone photoresist material and then exposed to theappropriate actinic energy source under the appropriate conditions toform a second set of lines that run orthogonal to the first set oflines. That is, if the first set of lines run in the x-direction, thenthe second set of lines will run in the y-direction and vice versa. Inthis manner, a grid-like pattern of resist is formed, leaving squareareas of substrate surface exposed. The substrate may then be processed,i.e., etched, implanted, etc., using the square mask for the pattern andthen the resist is removed to form a final product with square features.Care must be taken with the alignment of the first and second layers.When square images are printed using two separate reticles, an alignmentconcern for the alignment of subsequent mask patterns to this initialpattern is created: from the two initial masks, which one is chosen toalign to in subsequent processing? This creates a concern, becauseobtaining first-order alignment to the square image pattern in both Xand Y axes is often desirable. This concern can be addressed by the useof a specific alignment procedure at the subsequent levels. Thisprocedure involves viewing the alignment marks from both of the patternsused to make the square images. After collecting alignment data fromboth sets of marks, the software in the photo-tool can be used toextract the optimum X parameters from one mask level, and the optimum Yparameters from the next level, and use this information to establishthe optimum alignment parameters for the subsequent level. For aphoto-tool such as a Nikon, which uses two separate marks for X and Y oneach level, this would be easy to implement. For a photo-tool such as aMicrascan, which uses a single 45 degree chevron as the alignment mark,this information can be extracted, but requires that a software programbe written.

As shown in FIG. 13, a silicon wafer substrate 212 having an uppersurface 214 is provided. A first layer of polysilicon 220 (polySi) isdeposited on the upper surface 214 of the substrate 212. Then, a firstlayer of silicon nitride 230 is deposited on the upper surface of thepolySi 224. A first oxide layer 240, typically usingtetraethyl-orthosilicate (TEOS) is deposited on an upper surface of thefirst silicon nitride layer 234. These layers are chosen so there is anacceptable etch rate ratio between them.

As shown in FIG. 14, a second or thin layer of polySi 250 is thendeposited to a thickness of about 500 Å to about 1000 Å, and preferably750 Å on an upper surface of the first oxide layer 244. The second layerof polysilicon 250 is deposited to a thickness less than the first layerof polysilicon 220. A second or thin layer of silicon nitride 260 isdeposited to a thickness of about 500 Å to about 1000 Å and preferablyabout 750 Å on an upper surface of the second polySi layer 254 and thena second, thin layer of TEOS 270 is deposited to a thickness of about750 Å on an upper surface of the second nitride layer 264. The secondlayer of silicon nitride 264 is deposited to a thickness less than thefirst layer of silicon nitride and the thickness of silicon dioxide isin the range of about 500 Å to about 1000 Å. The second layer of siliconoxide is deposited to a thickness less than the first layer of siliconoxide. A first layer of resist (not shown) is deposited on an uppersurface of the second TEOS layer 274, exposed in lines running in eitherthe x-direction or the y-direction, and developed to expose portions ofthe upper surface of the second TEOS layer. As shown in FIG. 15, theexposed upper surfaces 274 of portions of the second layer of oxide 270are etched selectively to the upper surface 264 of the second layer ofnitride 260. The first layer of resist is then stripped and a secondlayer of resist (280 in FIG. 16) is deposited. The second layer ofresist is then exposed in a pattern with the lines running orthogonal tothose of the first layer of resist, i.e., in either the x-direction orthe y-direction, whichever the first layer of resist was not, anddeveloped, thereby exposing portions of the upper surface 274 of thesecond layer of oxide 270 and portions of the upper surface 264 of thesecond layer of nitride 260. The exposed portions of the second layer ofnitride 260 are then selectively etched, so that neither the secondlayer of polySi 250 nor the second layer of TEOS 270 are etched, therebyexposing portions of the upper surface 254 of the second layer of polySi250, as shown in FIGS. 16-18. As shown in FIG. 16, the resist runs inlines that are orthogonal to the lines created in the first etchingstep, which are the areas that have been etched down to the uppersurface of the polySi 254. As shown in FIG. 17, the areas that areetched down to the upper surface 254 of the second layer of polySi 250are the areas that no longer include any of the second, thin layer ofTEOS 270. As shown in FIG. 18, the thin layer of oxide 270 protects thesecond layer of nitride 260 from being etched, even in areas where theupper surface 274 of the second layer of oxide 274 is exposed, i.e., notcovered by resist 280.

As shown in FIG. 19, the resist is then stripped and the exposedportions of the second layer of polySi 250 are selectively etched,thereby exposing portions of the upper surface of the first layer ofTEOS 244. As shown in FIG. 20, the second layer of nitride 260 acts as amask material and is not etched. The first layer of TEOS 240 is also notetched. As shown in FIG. 21, the second layer of TEOS 270 is not etched.

As shown in FIG. 22, the first and second layers of TEOS, 240, 270, areselectively etched, thereby exposing the upper surfaces 234 and 264 ofthe first and second nitride layers, 230 and 260, respectively. As shownin FIG. 23, the upper surface 234 of the first layer of nitride 230 isexposed, in addition to the upper surface 264 of the second layer ofnitride 260, which was exposed in the previous step. As shown in FIG.24, the upper surface 264 of the second layer of nitride 260 is exposedin the areas that were previously covered by the second layer of oxide270 (shown in FIG. 19). At this point the grating pattern of squareshapes has then been formed, however, it is located above the surface inwhich it is necessary to form the grating.

The first and second nitride layers 230 and 260 are then etched down tothe first and second polySi layers, 220 and 250, respectively,underneath. As shown in FIG. 25, and in cross-section in FIGS. 26 and27, the upper surface 224 of the deposited polySi layer 220 is therebyexposed in areas. The top layer on the remainder of the surface is thesecond, thin layer of polySi 250. As shown in FIG. 26, wells 290 havebeen formed that have the upper surface 224 of the deposited layer ofpolySi 220 as a bottom. As shown in FIG. 27, the upper surface 254 ofthe second layer of polySi is exposed, however, the first layer ofpolySi 220 remains protected.

All polySi, both the first or deposited layer 220 and the second layer250, is then selectively etched to oxide, so that the first layer ofoxide 240 is not removed from the areas where it is desired, as shown inFIG. 28, and in cross-section in FIGS. 29 and 30. As shown in FIG. 29,areas of the wafer surface 214 are exposed for the remaining processingsteps, which are well known in the art, while other areas are covered bythe first layers of polySi, nitride and oxide, 220, 230, and 240.

EXAMPLE 3

In deep trench (DT) capacitor structures, it is desirable to providesquare shapes without foreshortening, especially as scaling and designconstraints force these features to become smaller and smaller. In orderto accomplish these goals, the present invention provides a thirdembodiment of a method for forming square shapes for these features.

In this third embodiment for the process for use with IT features, thefirst and second layers do not form a complete grating pattern. Anon-grating or “dog bone” pattern is used for the second pattern toguarantee square shapes. The pattern is formed in the following manner.

As shown in FIG. 31, a proposed 1 Gb PAD structure is depicted havingdeep trench (“DT”) structures 301 previously formed in a silicon wafer300. The DT's are formed in the usual manner, by etching them out,depositing a collar oxide layer 302, and then filling the trench withpolysilicon 303. An oxide plug 304 is formed at the surface of thepolysilicon 303. A layer of gate polysilicon 305 is on the surface ofthe wafer 300 and a layer of an oxide 306 covers the entire surface. Onan upper surface of the oxide 306 is a layer of nitride 307.

As shown in FIG. 32, a layer of organic material 330, for example,polyimide, is deposited on an upper surface 314 of the nitride 307. Thepolyimide is applied by a spin coating to a thickness in the range ofabout 0.4 microns (μm) to about 1 μm. It should be noted that thethicknesses of this layer and all subsequent layers are driven by etchrate ratios. It is then hard baked at a temperature in the range ofabout 170° Celsius (C.) to about 220° C. for about 1 hour. After thelayer of polyimide has been completely processed, a thin layer of anoxide 340, preferably TEOS, is deposited to a thickness in the range ofabout 500 Å to about 750 Å, preferably about 750 Å. Next, a layer ofsilicon nitride 350 is deposited on an upper surface 344 of the layer ofTEOS. A first layer of a resist material 360, which is preferably apositive tone type of resist, is deposited on an upper surface 354 ofthe layer of nitride. The resist is exposed and developed in a regulargrating pattern having lines running along either an arbitrary x-axis ory-axis. For discussion purposes, the lines are formed in the y-directionand the cross-sectional schematics are taken across a line such asacross line 34—34 in FIG. 33. Thus, portions of the upper surface 354 ofthe layer of nitride are exposed. Next, the nitride layer 350 is etchedselective to TEOS to form channels 370, as shown in FIG. 33. Then theresist is stripped.

A second layer of resist 380 is then deposited and the resultingstructure is defined to complete the support area printing. As shown inFIG. 35, the second pattern of lines in the second layer of resist 380on the exposed polyimide surface 334 along with the exposed nitridesurface 354 is a non-grating pattern that resembles a “dog bone” shape.The pattern must be printed so that the resist does not cover any of thepreviously opened areas that must eventually form the square shapes.However, a certain amount of laxity is allowed in the overlay of thefirst pattern to the second pattern because the edges of the resist mayfall anywhere within the nitride lines as long as the overlappingportion of the second shape is orthogonal to the previously etched edge.As shown in FIG. 36, the channels 370 have been enlarged, having theupper surface 334 of the polyimide layer as the bottom of the channel370 and sidewalls of oxide 342, nitride 352 and resist 382. The resistsidewalls 382 are pulled back from the oxide and nitride sidewalls 342,352, exposing portions of the upper surface 354 of the nitride layer,because of the laxity as discussed above.

The second layer of resist 380 is then stripped, and the organic layer330 is etched selective to the TEOS and nitride. This results in thestructure depicted in FIG. 37. As shown in FIG. 38, the channels 370have become deeper. The bottom of the channel is the nitride surface314, and the sidewalls are now organic sidewalls 332, TEOS sidewalls342, and nitride sidewalls 352. The isolation trenches may now be etchedby commonly known methods into the wafer surface using the organic layer330 as the mask. The usual etching process will strip the additionallayers of mask TEOS 340 and nitride 350 during the first phase of theetch. As shown in FIG. 39, the organic layer 330 is then stripped,resulting in the final structure 400. The photo process windows aresignificantly increased by utilizing this method, and there is full,independent control over the trench length and width.

EXAMPLE 4

Example 3 describes a process in which square-sided trenches are formedfor purposes of exposing portions of deep trench storage capacitors. Inthis Example 4, these same general techniques are used to form thestorage capacitor itself, as well vertical single-crystal silicon.pillar. structures on which vertical FETs can be formed. Because theprocess of the invention produces trenches and pillars that have squaredcorners, the effective capacitive storage area for each cell is greaterthan that of conventional trench capacitors having more rounded corners,without increasing the total surface area allocated to each storagecell. Moreover, more of the wafer area allocated for each cell can beused to form the cell itself, because the formed trenches and pillarsrequire less .spare. surface area to account for trench rounding. Forexample, for the 64 M DRAM described below, the total unit cell size canbe reduced from 9.3 .squares. (each .square. having dimensions near theresolution limit of the photolithographic tool) to below 8 squares.

With reference to FIG. 40, deep trench slots 510 having squaredsidewalls are formed in a substrate 500. The trench will be formed usingthe imaging and etch techniques described above for forming squaredimages in Examples 1-3. The trench slot will be formed as describedbelow, and after subsequent processing to define the cell elements, suchas devices and storage node, a second slot will be formed angularlyrelative to the first slot structure to form the final deep trenchstructures. By spacing the deep trench slots sufficiently closetogether, the invention forms square sided pillars 520 between the deeptrenches 510. In practice, the trenches 510 are 6-10 microns deep andare spaced from one another by a distance of 0.18 microns, such that apillar 520 is 6-10 microns high and 0.18 microns wide.

As shown in FIG. 41, a layer of silicon oxide/silicon nitride 511(shownin both dashed and solid lines) is formed to a thickness of 2500angstroms on the substrate 500 using conventional thermal growth andnitride deposition processes. A layer of doped polysilicon 512 (shown inboth dashed and solid lines) is then formed on the substrate, and isetched back (using a plasma etchant such as Cl₂ that etches polysiliconwithout etching the oxide/nitride dielectric) to remove the dashed lineportions. As a result, a portion of the oxide/nitride 511 is exposed.The exposed oxide/nitride 511 is etched in a plasma (such as CHF3) thatetches these dielectrics without substantially etching silicon, toremove the dashed line portions. As a result, the trench 510 ispartially filled with polysilicon 512A (to form the plate of a trenchcapacitor) with sidewalls having dielectric 511A forming the nodedielectric for the trench capacitors. Then a heating cycle is carriedout to drive n-type dopant out of polysilicon 512A, through thedielectric into surrounding portions of the silicon substrate, to formdiffusion areas 524 that serve as the storage node plate of the trenchcapacitors.

As shown in FIG. 42, a thick layer (on the order of 2-300 angstroms) ofisolation oxide 513 including portions 513A, 513B, and 513C is thendeposited on the substrate 500. This thick layer of oxide will providevertical isolation (along the side of pillars 520) between the storagecapacitor and the vertical FET. The wafer is then subjected to the sameplasma etchant as set forth above that is selective to oxide, to removeportion 513A of isolation oxide 513 on the horizontal surface of wafer500. Then a second layer of doped polysilicon 514 is deposited andetched using the same process as set forth above, exposing portion 513Bof the isolation oxide 513 while covering portions 513A. The exposedportion 513B is subsequently removed using the same oxide etchant usedpreviously, resulting in an isolating oxide .collar. structure 513C thatis covered with polysilicon 514A.

As shown in FIG. 43, a third layer of n-doped polysilicon 515 isdeposited and etched back (using the same process as defined above) tocreate a portion of polysilicon 515 that extends over the oxide collar513C. The wafer is then heated sufficiently to drive the n-type dopantfrom the doped polysilicon region 515 into the surrounding silicon,creating an n-type doped region 522 that will serve as one electrode ofthe vertical FET. A layer of oxide 516 (50-200 angstroms thick onpolysilicon, 20-80 angstroms on single crystal silicon, shown in bothsolid and dashed lines) is then grown on the wafer, covering the exposedsurface of polysilicon 515. A fourth layer of doped polysilicon 517(shown in both dashed and solid lines) is deposited, and is etched backusing the process described above to remove the dashed portions of layer517, leaving polysilicon 517A on portions 516B of the oxide layer. Thedashed portions 516A of the oxide layer are then removed. The oxidelayer 516B serves as the gate dielectric for the vertical FET. Note thatthe oxide layer 516B is thicker on the bottom (i.e. on the order of 150angstroms in thickness) to provide electrical isolation from theadjacent capacitor plate.

As shown in FIG. 44, the remaining structures of the vertical FET areformed. Oxide dielectric 526 is formed on the upper sidewalls of thetrench by depositing a layer of oxide and then carrying out a plasmaetch to remove horizontal portions of the layer, leaving only thevertical portions in the trench. Then a diffusion region is formed byimplanting n-type dopant into the exposed upper portions of the pillars520, to form bit line diffusions. To form the isolated pillars, a secondlithography step in the square imaging process is employed. In thisstep, a second slot is formed to define the length of the pillar,similar to the steps outlined in Examples 1-3. For example, the imagingtechnique set forth in FIGS. 32-38 of Example 3 (which were used toprovide deep trenches with square sides for exposing deep trench storagecapacitors) is used to define the pillars. The length of the pillar canbe 0.18 um. Following pillar definition, isolation is formed onalternate pillars by means of conventional lithography and isolationtrench methods. Appropriate metal contacts and layers are formed to (a)define word lines that interconnect adjacent polysilicon portions 517Aorthogonal to the plane of the FIGS. (i.e. into the page), and (b)define bit lines orthogonal to the word lines that interconnect adjacentbit line diffusions. Thus, by virtue of the invention, a vertical FET(having source diffusion BL, gate electrode 517A, and drain diffusion522) is formed that can access the information stored in a trenchcapacitor formed by polysilicon electrode 515, 514A, 512A, dielectric511A, and diffusion regions 524.

While the invention has been particularly shown and described withreference to preferred exemplary embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

Accordingly, what is claimed is:
 1. A method of forming cornered imageson a substrate comprising the steps of: (a) providing a substrate havinga first layer of selectively etchable material thereon; (b) forming aplurality of parallel edged openings in the first layer of etchablematerial, the openings aligned to form pairs of straight-edged firstregions; (c) depositing a layer of selectively etchable material overthe openings in said first layer of etchable material; (d) forming asecond plurality of parallel edged openings in the layer of patternablematerial, said second plurality of openings intersecting adjacent pairsof said straight-edged first regions forming a plurality of secondregions bounded by two edges of one of said first regions and two edgesof one of said second region; and (e) processing the substrate in thesecond regions defined by said first and second openings.
 2. The methodof claim 1, wherein the substrate includes: a blocking layer on a topsurface; and wherein the selectively etchable material is a layer ofhard mask material; and wherein the processing of the substrate includesremoving the blocking layer from the top surface in the region definedby the first and second openings.
 3. The method of claim 2, wherein theprocessing step includes etching the blocking layer and furthercomprising the step, after step (e) of: (f) stripping the second layerof patternable material.
 4. The method of claim 3 further comprising,after the second layer of patternable material is stripped, the step of:(g) over-etching the blocking layer for an amount of time sufficient toform rim-type phase shifters.
 5. The method of claim 1, wherein thesubstrate includes a second layer of selectively etchable materialbetween an upper surface of the substrate and a lower surface of thefirst layer of selectively etchable material, further comprising, afterstep (d) and before step (e), the step of: forming a plurality ofcornered openings in the second layer or selectively etchable materialin the second regions defined by the first and second openings.
 6. Themethod of claim 5, wherein the first layer of selectively etchablematerial is selected from the group consisting of: silicon nitride andsilicon oxide, and wherein the second layer of selectively etchablematerial is selected from the remaining of: silicon nitride and siliconoxide.
 7. The method of claim 1 wherein the substrate further comprises:a first layer of polysilicon on an upper surface of the substrate; afirst layer of silicon nitride on an upper surface of the first layer ofpolysilicon; a first layer of silicon oxide on an upper surface of thefirst layer of silicon nitride; a second layer of polysilicon on anupper surface of the first layer of silicon oxide; a second layer ofsilicon nitride on an upper surface of the second layer of polysilicon;and wherein the first layer of selectively etchable material is a secondlayer of silicon oxide on an upper surface of the second layer ofsilicon nitride; and further comprising, after step (d) and before step(e), the step of: (d1) forming a plurality of cornered openings in thesecond layer of silicon nitride in the second regions defined by thefirst and second openings.
 8. The method of claim 7, further comprising,after step(d1), the steps of: selectively etching the second layer ofpolysilicon to expose cornered areas of the first layer of siliconoxide; selectively etching the first layer of silicon oxide to exposecornered areas of the first layer of silicon nitride, thereby removingany remaining portions of the second layer of silicon oxide; selectivelyetching the first layer of silicon nitride to expose cornered areas ofthe first layer of polysilicon, thereby removing any remaining portionsof the second layer of silicon nitride; and selectively etching thefirst layer of polysilicon to expose cornered areas of the substrate,thereby removing remaining portions of the second layer of polysilicon.9. The method of claim 8, wherein step (e) comprises: processing theexposed cornered areas of the substrate to form deep trench capacitors.10. The method of claim 7, wherein the second layer of polysilicon isdeposited to a thickness less than the first layer of polysilicon,wherein the second layer of silicon nitride is deposited to a thicknessless than the first layer of silicon nitride; and wherein the secondlayer of silicon oxide is deposited to a thickness less than the firstlayer of silicon oxide.
 11. The method of claim 7, wherein the thicknessof the second layer of polysilicon is in the range of 500 Å to about1000 Å, wherein the thickness of the second layer of silicon nitride isin the range of about 500 Å to about 1000 Å and wherein the thickness ofsilicon dioxide is in the range of about 500 Å to about 1000 Å.
 12. Themethod of claim 1, wherein the second plurality of parallel edgedopenings is a non-grating pattern of lines having ends and wherein theends of the lines fall within remaining portions of the first layer ofphotoresist material proximate the two edges of the first regions. 13.The method of claim 1, wherein the substrate includes: a layer of gatepolysilicon on a surface of the substrate; a layer of an oxide on anupper surface of the gate polysilicon; a layer of nitride on an uppersurface of the layer of oxide; a layer of organic material on an uppersurface of the layer of nitride; a thin layer of an oxide on an uppersurface of the layer of organic material; and wherein the first layer ofselectively etchable material is a thin layer of nitride on an uppersurface of the thin layer of oxide; and further comprising, afterstep(d) and before step(e), the step of: selectively etching the thinlayer of an oxide in the plurality of second regions; removing remainingareas of the patternable material; selectively etching the layer oforganic material in the plurality of second regions; selectively etchingthe layer of nitride in the plurality of second regions, therebyremoving remaining areas of the thin layer of nitride; selectivelyetching the layer of oxide in the plurality of second regions, therebyremoving remaining areas of the thin layer of oxide; and selectivelyetching the layer of gate polysilicon, thereby exposing the substrate inthe plurality of second regions.
 14. The method of claim 13, wherein theprocessing of the substrate includes the steps of: non-selectivelyetching the substrate in the plurality of second regions using the layerof organic material as a mask; and stripping the layer of organicmaterial, thereby forming a final structure.
 15. The method of claim 13,further comprising, after exposing the substrate, the step of:processing the exposed second regions of the substrate to form isolationtrenches.
 16. The method of claim 13, wherein the second pattern oflines is a non-grating pattern.
 17. A method of exposing selectedportions of buried conductors, comprising the steps of formingconductive structures in a substrate, said conductive structures beingseparated from one another by first insulator structures and beingcovered with second insulator structures; forming a first gratingpattern of parallel openings in upper ones of said second insulatorstructures; and covering portions of said upper ones of said secondinsulator structures with a mask having a second non-grating pattern ofparallel openings crossing over to said first grating pattern, andetching areas of said first and second insulator structures defined bythe intersections of said first grating pattern and said secondnon-grating pattern to expose portions of said conductive structures.